Horizon OS propably adjusts the SoC clocks based if it's docked or not. Games on the other hand propably can't say whether its docked or not so it is checking for cpu clocks. If it 700MHz then docked, 200MHz then undocked. Propably forcing the cpu clock would trigger dock mode. If you use a powerful charger that has similar voltage to dock it might trigger docked mode too.
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Switch power consumption is 12W docked and 9W undocked. So it's already using lots of juice.
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Switch is only 5°C hotter while docked. Of course it is already hot when undocked so extra 5°C would certainly be noticeable.